L-Proline Catabolism in Gram-Positive Bacteria: Themes and Variations

The amino acid L-proline can be degraded to L-glutamate by many Gram-positive bacteria through a common pathway that uses the FAD-dependent activity L-proline dehydrogenase (PRODH) and the NAD+-dependent activity L-Δ1-pyrroline-5-carboxylate dehydrogenase (P5CDH), also called L-glutamate-γ-semialdehyde dehydrogenase (GSALDH).) The aim of this review is to summarize the processes of proline catabolism in the low G+C Firmicutes and the high G+C Actinobacteria and to illustrate the variations that commonly occur. In the low G+C clade, the two enzyme activities occur as single monofunctional proteins. In the high G+C clade, the two activities may occur either as monofunctional or bifunctional proteins. PRODH activity is a peripheral membrane-associated protein while P5CDH (GSALDH) activity may be found in the cytoplasm or associated with PRODH in the membrane fraction. The only major exception occurs in anaerobic Clostridium species which carry out cytoplasmic two-step Stickland reactions. An important variation in proline catabolism among these microorganisms is in their regulation. Some bacteria in the low G+C clade such as Bacillus species have a specific regulatory protein that binds to DNA in the presence of L-proline and activates transcription. Other bacteria in this clade such as Staphylococcus species show no regulation of proline catabolism. Some bacteria in the high G+C clade such as Mycobacterium species also have a specific proline regulatory protein. In addition, L-proline catabolism may be affected by the global nitrogen and carbon regulators CodY and CcpA. However, CodY only appears to occur in the low G+C clade of Gram-positive bacteria. While carbon catabolite repression occurs in both clades, the protein CcpA is found in only some bacteria in the low G+C clade. Because a large amount of data is now available about various Gram-positive bacteria, further studies on the biochemistry, genetics, and physiology of L-proline degradation in these microorganisms may help explain the significance of these variations.